Currently, dynamic random-access memory (DRAM) metal-insulator-metal capacitors typically use electrodes made of TiN. For future scaling of DRAM metal-insulator-metal capacitors, though, it may be desirable to use a metal electrode that does not contribute to the Equivalent Oxide Thickness (EOT) due to the oxidation of its top surface. One option, then, is to replace the TiN electrodes with electrodes made of ruthenium (Ru). Even when oxidized at the top surface, RuO2 will show metal-like conductive behaviour and, as such, not contribute to the EOT. The electrodes may be deposited by Atomic Layer Deposition (ALD) or Chemical Vapour Deposition (CVD), so as to ensure good step coverage in high aspect ratio structures
Further, for future scaling of DRAM metal-insulator-metal capacitors, it may be desirable to use a dielectric with a k-value higher than the k-value given by Al2O3 (e.g., around 9) and/or ZAZ (ZrO2/Al2O3/ZrO2) (e.g., around 40). The dielectric may, like the electrodes, be deposited by ALD or CVD to ensure good step coverage in high aspect ratio structures. ALD of high-k dielectric layers often uses ozone (O3) as oxidant.
The k-value of the high-k dielectrics often depends on the crystalline structure of the material. TiO2 in its rutile form has been reported to have a k-value of about 80, while only about 40 in its anatase phase. It is, however, not easy to grow TiO2 in its rutile phase by ALD. It prefers to form anatase TiO2, and only after high temperature anneal will anatase TiO2 transform to rutile TiO2. Commonly, rutile TiO2 is only obtained above 750° C. However, in many metal-insulator-metal capacitor applications, anneal temperatures cannot exceed 650-700° C., due to the susceptibility to oxidation of the electrodes.
TiO2 as grown on TiN by ALD is found to be either amorphous or crystallized in the anatase phase. The phase is maintained up to at least 700° C. When O3 is used instead of H2O as oxidant at similar process conditions (temperature), crystalline phase formation during deposition is favoured, since O3 is a stronger oxidant. For that reason, O3 may be preferred as oxidant. It was also reported that, when using Ru as bottom electrode, rutile TiO2 was obtained on Ru when using O3 as oxidant in the ALD TiO2 process at 250° C. as described by, for example, Kim et al., “High dielectric constant TiO2 thin films on a Ru electrode grown at 250° C. by atomic-layer deposition,” Appl. Phys. Lett., 85, 4112 (2004).
Rutile phase TiO2 can be obtained when using a crystalline template approach, such as tetragonal RuO2, due to an excellent match in crystal structure between rutile tetragonal TiO2 (lattice parameters: a=b=4.59 Å, c=2.96 Å) and tetragonal RuO2 (lattice parameters: a=b=4.50 Å, c=3.10 Å). This is has been reported by, for example, Fröhlich et al., “Epitaxial growth of high-k TiO2 rutile films on RuO2 electrodes,” Electrochem. Solid-State Lett., 11, G19 (2008).
However, it has also been reported that O3 and Ru are not compatible, as described by, for example, Nakahara et al., “Etching technique for ruthenium with a high etch rate and high selectivity using O3 gas,” Journal of Vacuum Science And Technology B, 19, 2133 (2001), which asserts that Ru is instantly etched when Ru is exposed to O3. However, RuO2 is known to be much less reactive with O3.
Thus, without protection, Ru substrates are not compatible with O3 based high-k deposition processes. Practical etching and roughening of the surface will take place at the initial stage of the high-k deposition. Similar problems may occur with metals other than Ru. Also when the metal is not etched by O3, O3 exposure will oxidize the metal and if the metal oxide is not metal-like, it will give rise to an EOT contribution and higher resistivity.
Park et al., U.S. Patent Application Pub. No. 2009/0134445, describe a metal-insulator-metal capacitor structure including a bottom electrode obtained by ALD or CVD deposition of Ru, and a dielectric comprising a TiO2 layer in rutile phase. Between the Ru electrode and the TiO2 layer, an Al2O3 layer is provided, obtained by ALD and with O3 or H2O as oxidant. The function of the Al2O3 layer is to prevent deterioration of the interfacial characteristics between the bottom electrode and the TiO2 layer and to prevent oxidation of the bottom electrode. The TiO2 layer is obtained by ALD using O3 as the reactive component.
However, the Al2O3 does not form an optimal template for growing TiO2 in rutile form. Accordingly, after ALD deposition of the TiO2 layer, an oxidation plasma treatment or oxygen ion beam irradiation treatment is required for changing portions of TiO2 which had not grown in rutile form. Also, Al2O3 has a moderate k-value of about 9, and the protective layer will give an unacceptable contribution to the EOT.
Another problem in existing techniques is that low roughness of the Ru layer is not maintained when Ru/RuO2 is produced by oxidation of the Ru layer. Current oxidation techniques for obtaining an RuO2 layer on top of the Ru (e.g., oxidation by O3 treatment or subjecting the Ru layer to an O2 anneal) tend to increase the roughness of the substrate, because of unwanted etching by O3 and/or because of the localized formation of RuO2 crystals. There is currently no oxidation technique for Ru which preserves the smoothness of the Ru-surface.